THE SEARCH of DARK MATTER with Ardm DETECTOR∗ ∗∗

Vol. 37 (2006) ACTA PHYSICA POLONICA B No 7

THE SEARCH OF DARK MATTER WITH ArDM DETECTOR∗ ∗∗

Piotr Mijakowski

The Andrzej Sołtan Institute for Nuclear Studies Hoża 69, 00-681 Warsaw, Poland

(Received May 8, 2006)

The ArDM project aims at developing and operating a 1 ton-scale liquid argon detector for direct detection of Weakly Interacting Massive Particle (WIMP) as Dark Matter in the Universe. In the ﬁrst part of this paper the main features of the detector are presented. The second part includes a discussion on expected experimental background.

PACS numbers: 13.75.–n, 28.20.Cz, 95.35.+d

1. Introduction

There are many evidences that ordinary baryonic matter compose only ∼4% of the matter in the Universe. The rest is dominated by Dark Energy (∼73%) and Dark Matter (∼23%). The latter one presumably comprises of a new type of elementary particles. Understanding of its nature is a key issue in the contemporary particle physics. One of the promising candidate is the Weakly Interacting Massive Particle (WIMP), introduced in some SUSY models. There are worldwide efforts aimed to detect these particles. It seems to be clear that future Dark Matter experiments will need very high sensitivity detectors (big target mass and high background discrimination powers) to explore effectively the region of WIMP parameters and achieve sufficient counting rate.

∗ Presented at the Cracow Epiphany Conference on Neutrinos and Dark Matter, Cracow, Poland, 5–8 January 2006. ∗∗ Supported partially by the Polish State Committee for Scientiﬁc Research, SPUB 621/E-78/P-03/DZ214/2003-05.

(2179) 2180 Piotr Mijakowski

2. ArDM detector In 2004, interested groups started a new initiative for direct detection of Dark Matter — Argon Dark Matter experiment, ArDM1. The goal of this project is to design, assemble and operate a dual-phase ≈1 ton argon underground detector, to demonstrate the feasibility of a noble gas ton- scale experiment in terms of eﬃcient performance and suﬃcient background discrimination capabilities. The experimental technique is based on detecting tiny recoils of argon nuclei induced by the collisions of Dark Matter particles with the detector target. Deposited energy leads to the production of scintillation light and ionization charge in the medium. Typical kinetic energy of recoils is in the range of 10–100 keV. The signal is therefore quite elusive and requires very good background rejection. In addition, due to the very small WIMP-nucleus interaction cross section, a very rare event is expected. The conceptual layout of the detector is shown in Fig. 1. More detailed information can be found in [1]. One of the key features is to independently

Fig. 1. Illustration of ArDM experiment and WIMP detection principle. detect the primary (scintillation) and secondary (charge) signals. The light produced in the scintillation and recombination of free electrons will be detected by a light readout system located on the bottom of the detector behind the transparent HV cathode. To increase the light collection eﬃ-

1 See: http://neutrino.ethz.ch/ArDM. ETH Zürich, CIEMAT, Granada University, Sheffield University, Soltan Institute Warszawa, Zürich University. The Search of Dark Matter with ArDM Detector 2181 ciency, primary VUV scintillation light of argon (128 nm) will be reflected by specially conceived high reflectivity mirrors placed around the field shap- ing electrodes. Ionizing charges will be drifted towards the top of the detector, extracted to the gas phase and amplified there by the Large Electron Multiplier (LEM) system. By segmenting the LEMs it is possible to obtain a 2 dimensional image of an event. The time correlation between scintillation and charge could provide the information on the third coordinate, thus allowing a full spatial event reconstruction. The ratio of the primary to the secondary signal allows to distinguish effectively between heavy recoils and other backgrounds. The time dependence of scintillation light can be used further to provide additional possibility of background discrimination. The immediate plan of ArDM group is to setup and operate a 1 ton prototype at CERN. R&D efforts are under way to acquire and construct all crucial parts of the detector: LEM based charge amplification and readout, light detection system, cryogenics, drift volume, LAr purification and HV system. Assuming the successful operation of the prototype, the group is considering a deep underground operation at the Canfranc Underground Laboratory.

Fig. 2. Cross-section normalized to nucleon versus WIMP mass. The expected ArDM event rates for a true recoil energy threshold of 30 keV and certain cross- section values are indicated with crosses. 2182 Piotr Mijakowski

The sensitivity expectation of the ArDM 1 ton prototype is shown in Fig. 2. Expected event rate for a true recoil energy threshold 30 keV, WIMP mass of 100 GeV and WIMP-nucleon cross-section of ∼10−42 cm2 (10−6 pb) is 100 events per day per ton. Providing that suﬃciently low gamma and neutron background levels can be reached, this sensitivity would increase to ∼10−8 pb. For the year of operation it would results in ∼10−10 pb.

3. Experimental background The background particles, consisting mainly of electrons, photons, neutrons, helium nuclei, muons and neutrinos could produce nuclear and electron recoils inside the detector. The dominant electron/gamma background leads to the production of electron recoils and could be rejected by the ratio of the scintillation to the ionization yields, which is much lower in that case than for a nuclear recoils expected also for a WIMP interactions. However, high discrimination power (> 109) is needed in the experiment using LAr due to the presence of 39Ar beta-emitter in natural argon liquefied from the atmosphere. 39Ar isotope decays with a half life of 269 years and a value Q = 565 keV. Its activity in LAr has been measured [2] and is expected to induce a background rate of ≈ 1 kHz in a 1 ton detector. The alternative way to suppress it, is to obtain 39Ar-depleted targets by extracting argon from well gases rather than from the atmosphere [1]. Providing that electron/gamma background rejection is sufficiently high, this type of background does not limit the detector sensitivity. The nuclear recoils induced by muons can be usually tagged by active veto, and events associated with neutrinos are reported to be negligible [3]. Alpha particle interactions usually can be rejected as they deposit a few MeV energy in the target and their keV interactions occur only near the vessel walls, thus could be eliminated by a good fiducial volume definition. Multiple elastic scattering of neutrons is also a clear indication for the background events, as WIMPs, due to the weak coupling, do not undergo multiple interaction in the detector. Only the single elastic scattering of a low energy neutrons would be indistinguishable from expected WIMP scattering. That provides a solid argument for a careful neutron background studies. There are two possible sources of neutrons in deep underground locations: either they are produced in cosmic-ray muon interactions, or by natural radioactivity. The first ones are induced by the high energy through- passing muons in the rock and elements surrounding the detector. Neutrons from local radioactivity are cased by U/Th traces in the rock and detector components. They are produced in spontaneous fission of 238U or via (α,n) reactions initiated by α’s from decays of radioactive isotopes in U/Th chains. The Search of Dark Matter with ArDM Detector 2183

Following the approach presented in [4] we can consider three classes of neutron sources: (1) neutrons from radioactivity in surrounding rock, (2) neutrons from radioactivity in detector components, (3) muon-induced neutrons. Energy spectra, flux, place of the origin and methods of their suppression are different in each case. The overall flux of neutrons from the surrounding −6 −2 −1 rock is expected to be the most dominant (φrock = 3.8×10 cm s at the Canfranc site [5]) but it can be effectively reduced by the hydrocarbon shielding. The flux of neutrons from detector components depends strongly on the choice and radiopurity of the detector materials. This type of background is considered to be the most difficult to reject as it cannot be suppressed by any shielding and it is also not possible to estimate its flux and energy spectrum with high accuracy. Contamination of U/Th cannot be measured for all detector components, it shows a variation between the samples of the same kind, and converting it to the resulting neutron flux is not an easy task and requires often some simplifying assumptions. In the ArDM experiment the glass parts of PMTs, glass fibres in Vetronite plates and capacitors are recognized as the major internal source of neutrons. The mean energy of neutrons associated with the radioactivity is typi- cally in the order of 1–2 MeV, depending on the composition of the material. The energy spectrum of muon-induced neutrons is hard, extending to GeV scale. This results in the following consequences: muon-induced neutrons can reach the detector from large distances, they produce higher energy recoils and can easily penetrate through the shielding. The flux of muon- induced neutrons φµ−ind is usually of a three orders of magnitude lower than −9 −2 −1 φrock for various underground locations (φµ−ind = 1.7 × 10 cm s at the Canfranc site [5]). Very often events induced by these neutrons can be tagged using the active veto system and information about the time correlation with the passing muon. Investigations on neutron background sources and simulations on their propagation and interactions inside the detector are being performed within the ArDM group. Their purpose is to evaluate the expected number of events with a single neutron scattering in the detector what will help to specify requirements for veto system, shielding and purity of the detector materials. The author’s Diploma thesis [7] concerned the neutron background studies for liquid argon Dark Matter detector. First stage of presented work consists of a set of tests and verifications of the low energy neutron physics applied in the developed simulation program. Neutron interactions in LAr 2184 Piotr Mijakowski

Elastic scattering on 40Ar (incident neutron energy = 2 MeV)

900 bin size = 1.00 keV

800

700

600

500

400

number of events 300

200

100

0 0 20 40 60 80 100 120 140 160 180 200 Argon true recoil energy [keV]

Fig. 3. Simulated 40Ar recoil spectra. Elastic scattering of 2 MeV neutrons in liquid argon. were simulated using the Geant4 toolkit [6]. Mainly the neutron elastic scattering and neutron capture processes were investigated as they contribute the most in the low energy range. The example of a true recoil energy spectrum of argon nuclei obtained for the 2 MeV incident neutrons is shown in Fig. 3. Thereafter, some preliminary results on the rock neutrons were discussed. Assuming the ﬂux φrock reported for the Canfranc site at the walls of the ﬁducial volume, one can expect ∼ 10000 incoming neutrons per day. Simulations showed that more than a half of them would interact inside the detector and half of the interacting ones would scatter more then once. Some of the multiple scattering events however could not be recognized due to limited spatial resolution of the detector, what will slightly increase the number of seen single recoil (WIMP-like) events. After assuming 2 cm detector resolution, we are left with several thousands WIMP-like events per day. To reach acceptable level of high sensitivity required for a successful WIMP detection, one should further reduce the number of a WIMP-like events by 104–106 orders of magnitude by using an external neutron shielding. Similar simulations were also performed for neutrons from the detector components, showing that roughly 70% of the interacting ones can be recognized in the experiment as multiple scattering events. These studies are presently continued in the collaboration.

4. Outlook The plan of the ArDM group is to develop and operate a dual-phase argon detector for direct detection of WIMPs with independent ionization and scintillation readout. With a 1 ton prototype the validity of this design The Search of Dark Matter with ArDM Detector 2185 should be shown. The ﬁrst goal would be to understand and control the detector performance and a proof of principle test on electron background (39Ar) rejection vs. nuclear recoils.

The help of all ArDM colleagues from ETH Zürich, CIEMAT, Granada University, Sheﬃeld University, Sołtan Institute and Zürich University, is greatly appreciated.

REFERENCES

[1] A. Rubbia, Talk given at 9th International Conference on Topics in Astropar- ticle and Underground Physics (TAUP), Zaragoza (Spain), 10–14 Sep. 2005. [2] A. Szelc, Talk given at Cracow Epiphany Conference on Neutrinos and Dark Matter, Cracow (Poland), 5–8 Jan 2006. [3] R. Brunetti et al. [WARP Collaboration], Experiment Proposal, 2004, http://warp.pv.infn.it/ [4] M.J. Carson et al., Astropart. Phys. 21, 667 (2004). [5] J.M. Carmona et al., Astropart. Phys. 21, 523 (2004). [6] Geant4 Collaboration, http://geant4.web.cern.ch/geant4/ [7] P. Mijakowski, Diploma thesis, (in polish), available at http://neutrino.fuw.edu.pl/public/msc/